Recently, display devices such as liquid-crystal displays (LCD), and input devices to be combined with the display devices such as touch panels have become widely used in various fields. In producing such display devices and input devices, transparent pressure-sensitive adhesive sheets (tapes) are used for attaching optical members. For example, for attaching a touch panel to the optical member (e.g., protective plate) of various display devices, a transparent pressure-sensitive adhesive sheet is used (for example, see JP-A-2003-238915, JP-A-2003-342542 and JP-A-2004-231723).
With the expansion of the applicability of those display devices and input devices, the pressure-sensitive adhesive sheet for use in such devices has become required to satisfy not only transparency but also the ability to fully show its potency as a pressure-sensitive adhesive sheet in various environments. For example, the pressure-sensitive adhesive sheet is required to satisfy adhesion reliability at high temperatures, for example, having excellent adhesiveness and resistance to foaming or peeling (foaming/peeling resistance) in high-temperature environments. In addition, the pressure-sensitive adhesive sheet is further required to satisfy the property (visibility and outward appearance retentivity) of not having any negative influence on the visibility and the outward appearance of the attached optical members and optical articles in various environments (for example, in high-temperature high-humidity environments).
As a pressure-sensitive adhesive sheet excellent in visibility and outward appearance retentivity, there is proposed a pressure-sensitive adhesive sheet not getting cloudy by humidification (see JP-A-2004-263084). However, though excellent in visibility and outward appearance, the pressure-sensitive adhesive sheet is problematic in that its adhesion reliability at high temperatures is poor, for example, its adhesion force lowers at high temperatures therefore causing foaming or peeling. Specifically, a pressure-sensitive adhesive sheet, which satisfies both visibility/outward appearance retentivity and high-temperature adhesion reliability, has not been obtained yet.